The present invention relates in general to multiple address radio systems and, more particularly, to a multiple address radio system having automatic radio frequency calibration.
In earlier multiple address systems between the central station and the remote radio sites, technicians were required to visit remote sites for calibration and adjustment of the frequency offset of the remote radio at the remote radio sites.
Temperature compensated crystal oscillators have been employed in the transmission of radio frequency signals from a central station and from the remote radios at the remote radio sites. Over a period of time, temperature compensated crystal oscillator frequencies may vary. The variations in the oscillation frequencies for temperature compensated crystal oscillators are not consistent, or the same. Therefore, the output radio frequency of the central station and the output radio frequency of a remote radio at a remote radio site may vary. In narrow band channels, a radio frequency offset between a central station and a remote radio at a remote radio site tend to degrade data communication quality.
Remote radios at remote radio sites have employed automatic radio frequency control. In the use of automatic radio frequency control, a voltage at the remote radio site is sampled and temporarily held in order to change the bias voltage of the temperature compensated crystal oscillator. The bias voltage momentarily alters the output radio frequency of the temperature compensated crystal oscillator. For a remote transceiver radio which changes between a receive state and a transmit state continuously while being polled, a delay in the automatic radio frequency control can increase the bit error rate. At fringe areas in which the signal strength from the master station is weak, a remote transceiver radio using automatic radio frequency control may tend to track and lock onto an incorrect central station.